In an image forming apparatus, such as a copy machine, a facsimile machine or a printing machine, an electrostatic latent image is formed by a light on a photoreceptor and the electrostatic latent image is developed into a toner image by a developing device. Then the toner image is transferred as a visualized image onto a sheet so as to output recorded information.
There are two types of developer used in the development process. One is magnetic or nonmagnetic single-component developer and the other is two-component (binary system) developer containing toner and carrier for carrying toner particles. When these components are mixed, toner particles are electrically charged due to frictional electrification caused by the stirring/mixture process and become attached to the electrostatic latent image via electrostatic attraction.
A known structure of a developing device is a combination of a developing sleeve and a stirring sleeve. The developing sleeve produces particle clusters along its circumferential surface to supply the toner particles onto an electrostatic latent image on a photoreceptor. The stirring sleeve supplies the stirred and mixed developer to the developing sleeve. After the visualization of the electrostatic latent image on the photoreceptor, the remaining developer from which toner particles have been used is collected in the developing device.
The two-component developer is stirred and mixed in a developer tank, during which process the toner particles are electrically charged. If electrification is insufficient, the image density is adversely affected and the image quality is likely to be unstable. Especially if the electrification level is less than a predetermined level, the image density cannot be maintained at a desired level. In addition, toner particles are suspended and attached to the background surface of the photoreceptor, which phenomenon is called background contamination. If the electrification level is over the predetermined level, too large an amount of toner particles adhere to the image, which results in a so-called covered image.
To overcome the above-described problems, it is proposed to provide a stirring part separated from the developing device and supply the stirred developer to the developing device by means of a feed screw (See, for example, Patent Document 1). Another known structure is to provide circulating means to connect a developing part and a stirring part that is adapted to supply toner particles to the developing part making use of air current (See, for example, Patent Document 2). In the latter document, the stirring part performs the stirring process in accordance with the state of the developer to supply a developer with an appropriate toner concentration and electrification level to the developing part.
Thus, feed screw means or air-draft feeding means are employed in the conventional techniques to supply stirred and mixed developer from a stirring part to a developing device. However, the quantity of developer supplied using the known techniques depends on the rotational speed of a screw or a rotary feeder having rotatable blades arranged in a radial fashion along the circumference of the port, and the feeding quantity may vary according to aging variation of the developer.
If the fluidity of the developer degrades due to the environmental conditions or long-term use, the feeding quantity will vary. If the developer is fed by a feeding screw in a direction against gravitational force, the volume of the developer being fed will change depending on the toner concentration. Some portions of the developer may be fed back in the opposite direction. Thus the feeding quantity of the developer cannot be maintained constant.
Still another problem caused by use of a screw or a rotary feeder is that an air gap is generally provided between the inner wall of the housing and the screw or the rotary feeder. For this reason, the sealing characteristic may be insufficient, and this may cause the feeding quantity to be inconsistant.
Air-current feeding of the developer may be desirable compared with use of a screw; however, it is difficult for the air-current feeding method to check the actual quantity of the developer being fed in the developing device, especially the quantity of the developer fed to the developer supply port although a rotary feeder is able to define a quantity of feeding. Furthermore, the quantity of developer introduced in the developing device may vary depending on a change in the environmental conditions such as humidity or a change in the feeding conditions such as a feeding volume.
If the quantity of developer supplied to the developing device is short of a required quantity, the image density become unstable. If too great a quantity of developer is supplied to the developing device, the developer will overflow and scatter in the surroundings.    Patent Document 1: JP H04-198966 A    Patent Document 2: JP 2008-3561 A